This invention is directed to a method and system for treating a patient's heart with therapeutic or diagnostic agents. More particularly, it involves a means to form channels in desired layers of the heart muscle, the epicardium, endocardium and myocardium, and means to deliver therapeutic or diagnostic agents into the channels.
Targeted delivery of therapeutic or diagnostic agents is a desirable but often difficult goal. Potential benefits include efficient use of the agent and limitation of agent action to the desired area. However, the problems that must be overcome are significant: access, transporting the agent to the desired area of the patient; minimization of systemic loss, keeping the agent within the desired area; and timing, ensuring a sufficient quantity of the agent is available in the desired area for sufficient period of time to achieve the therapeutic or diagnostic effects.
One promising strategy for agent delivery involves somatic gene therapy. Cells in a desired region of the body are engineered to express a gene corresponding to a therapeutically or diagnostically useful protein. Genetic information necessary to encode and express the protein is transferred to the cells by any of a number of techniques, including viral vectors, electroporation, receptor-mediated uptake, liposome masking, precipitation, incubation and others. Gene therapy can be a direct in vivo process where genetic material is transferred to cells in the desired region of the patient's body. Most current in vivo strategies rely on viral vectors. Alternatively, the process can be an indirect in vitro process where cells from the desired region are harvested, genetic material is transferred to the cells, and the cells are implanted back in the patient's body. In vitro techniques allow for more flexibility in transfer methods and may be safer since viral vectors need not be introduced into the patient's body, thus avoiding the theoretical risk of insertional mutations, replication reactivation and other harmful consequences. However, not all tissues are susceptible to harvesting and implantation and require an in vivo technique. The engineered cells can secrete the protein for a significant period of time, ensuring its supply in the target region. Human adenosine deaminase was expressed in vivo by rat vascular smooth muscle cells for over six months. Lynch CM et al., Proc. Natl. Acad. Sci. USA 89: 1138-42 (1992).
One region of interest for gene therapy is the circulatory system. Researchers have transferred genetic material to the vascular walls, particularly the smooth muscle and endothelial cells. Suitable delivery techniques include ligation of the vessel (Lynch et al., supra.), dual-balloon catheters (Leclerc G et al., J. Clin. Invest. 90: 936-44 (1992)), perforated balloon catheters (Flugelman M Y et al., Circulation 85: 1110-17 (1992)), stents seeded with transduced endothelial cells (Dichek DA et al., Circulation 80: 1347-53 (1989)) and vascular grafts lined with transduced endothelial cells (Wilson J M et al., Science 244: 1344-46 (1989).
However, these methods have not been found suitable for treatment of the heart muscle. Thus far, experimental gene therapy in rats has been achieved through direct injection of DNA into the myocardium. Lin H et al., Circulation 82: 2217-21 (1992) and Acsadi G et al., New Biologist 3: 71-81 (1991). In these studies, direct injection caused inflammation, apparent myocyte necrosis and scar tissue along the needle tracks. When compared to injection of plasmid DNA, gene transfer by injection of adenovirus vectors was markedly more efficient. Guzman RG et al., Circulation Research 73: 1202-7 (1993). Gene transfer using adenovirus vectors injected into pig hearts was highly efficient in regions immediately adjacent the injection, but evidence of gene transfer was found only within 5 mm of the injection. French B A et al., Circulation 90: 2414-24 (1994). As in the studies above, a prominent inflammatory response was associated with the injection. There remains a need for effective gene therapy methods for the heart.
Another difficulty associated with gene therapy is the need to transfer an effective amount of the genetic material in a clinically relevant time period. Exemplary techniques for introduction of engineered endothelial or smooth muscle cells or for in vivo gene transfer require total occlusion of the vessel for 30 minutes. Nabel E G et al., Science 249: 1285-88 (1990); Nabel E G et al., Science 244: 1342-44 (1989); and Plautz G et al., Circulation 83: 578-83 (1991). These time frames would not be feasible for delivery involving the heart. A study attempting to shorten these times employed a perforated balloon catheter and successfully delivered retroviral vectors within one minute, but achieved fewer than 100 transduced cells in a two cm segment of tissue. Flugelman et al., supra. Accordingly, there remains a need to provide gene therapy methods that effect sufficient cellular transduction either by providing more rapid transfer rates or by allowing long-term delivery without impermissibly interfering with cardiac function.
Targeted agent delivery that does not rely on gene therapy would also benefit from similar features. It is often desirable to release the therapeutic or diagnostic agent over a period of time. Levy R J et al., WO 94/21237 discloses a system and method for treatment of arrhythmia that involves transmyocardial delivery of time-release antiarrthymic agents by contacting the epicardium, endocardium or pericardium. Levy et al.'s drug compositions generally comprise a biocompatible polymer formulated to release the active agent in a controlled manner, preferably in the form of a patch applied to the exterior of the heart. The reference also suggests various intravascular placement methods including an implantable catheter tip, an expandable system with anchoring prongs or intramyocardial placement via a stab wound with a trocar. Thus, there is also a need for a system for cardiac agent delivery that effectively delivers agent to the heart wall.